Driven by continually shrinking feature sizes and adoption of ever more fragile materials in integrated circuit (IC) manufacturing, it has become crucial to develop effective and low impact processes that are benign to features on semiconductor wafers. Rinsing the wafers with carbonated deionized (DI—CO2) water is an example of a low impact process that may allow for damage free cleaning. There is thus a continuing interest in using gasified DI water in photolithography, wet etch and clean, and chemical-mechanical planarization (CMP) applications in semiconductor fabrication. One major challenge is how to produce and maintain water with low concentrations of a dissolved gas, since it is difficult to control the doping of water with small amounts of the dissolved gas.
Membrane contacting technology has been used to deliver high dissolved gas concentrations in liquids such as water. There are several other common practices used to make low concentration gasified solutions. A first method is to mix or dilute a desired gas with an inert gas such as nitrogen (N2) before injecting the gas mixture into the membrane contactor. The inert gas dilutes the concentration of the desired gas inside the membrane contactor, which leads to a low level of gas being dissolved in a liquid such as water. The target concentration of the gas dissolved in the liquid can be maintained by varying the flow ratio of the desired gas and the inert or carrier gas. This method can use large amounts of gas(es) to achieve a suitable dilution and therefore can be expensive and/or wasteful.
In a second method, high concentration gasified water is mixed or diluted with ungasified DI water in ratios to attain a desired low concentration of target gas in the liquid. Target concentrations of gas in the liquid can be maintained by varying the flow ratio of the high concentration gasified water and the ungasified DI water. This method can require large amounts of liquid(s) and can also be expensive and/or wasteful.
Examples of these methods can be found in the following patent documents. U.S. Pat. No. 6,328,905 discloses residue removal by CO2 water rinse in conjunction with post metal etch plasma strip. U.S. Pat. No. 7,264,006 discloses ozonated water flow and concentration control apparatus and method. U.S. Pat. No. 7,273,549 discloses a membrane contactor apparatus which includes a module having hollow fiber membranes. U.S. Patent Application Publication No. 2008/0257738 A1 discloses mixing CO2 and DI water in a chamber of a contactor that is filled with tower packing polymers with a high surface area per volume.
Although the first and second mixing or dilution methods may produce low dissolved gas concentration, each method has its own shortcomings. For example, mixing a desired gas with an inert gas or carrier gas may introduce other gases into the liquid which may be unnecessary contaminants in the process and would increase the total gas use for the process. Moreover, dissolving additional carrier gas in the liquid may increase the total gas concentration in water which can lead to undesirable and/or harmful bubbles. In addition, diluting high concentration gasified water uses extra water and adds complexity in system design and control which increase costs. What is more, condensation of liquid on the contactor surfaces can occur in both methods. If this condensation is not removed, the condensate could block the membrane and reduce the effective contacting area, leading to loss of performance efficiency and an inconsistency in the amount of dissolved gas in the liquid. As a result, frequent purge cycles are commonly used for the above two methods to remove the condensate, adding cost, downtime, and complexity to the system.